Lithographic printing plate with polymer coated metal image



Aug- 3, 1965 B. w. Nr-:HER 3,198,110

LITHOGRAPHIG PRINTING PLATE WITH POLYMER COATED METAL IMAGE OriginalFiled June l5, 1959 I NVEN TOR.

www

3,193,110 v MTHUGRAPH C PRHNTENG PLATE tt/ETH POLYMER CATED METALlli/HAGE Byron W. Naher, Hudson, Wis., assigner to Minnesota Mining andManufacturing Company, St. Pani, Minn., a corporation of DelawareOriginal application .inne l5, 1959, Ser. No. 820,254, new Patent No.3,127,331, dated Mar. 31, 1964. Bivided and this application Apr. 19,1963, Ser. No. 279,349

2 Claims. (Ci. lili-449.2)

This application is a division of U.S. Serial No. 820,- 254, now U.S.Patent No. 3,127,331.

This invention relates to a novel process for reproducing visibleimages. In one aspect, this invention relates to a method for theelectrolytic destruction ofthe rectification effect of selected areas ofphotoconductive material bonded to an electrically conductive backing.In another aspect, this invention relates to a nevel process forreproducing visible images on a photosensitive sheet. Still y quired,and the copy produced needs no further heating or other processing torender the image permanent. Brietiy, the copy sheet comprises aphotoccnductive powder, such as zinc oxide, bonded to a contiguouselectrically conductive backing. Since the photoconductive coating showsa rectification effect, i.e. allows current to pass essentially in onlyone direction, it is more effective to use the electrically conductivebacking sheet as the cathode in the electrolytic development process.With thebacking sheet as anode, the electrical resistance of thephotoconductive material is high, even in wet light rstruck areas.

Because of the rectification effect of photoconductors such as zincoxide, on a conductive backing sheet, the electrolytic development ofthe light struck photoconductive areas has been generally restricted tothe use of those materials which are reduced at the cathode to produce avisible image. Many oxidizable developers and materials bearing anegative charge have not heretofore been efliciently useable in theabove-described process.

It is therefore an object of this invention to provide a reverse currentprocess for the development or" an eX- posed photoconductive image.

Itis another object of this invention to provide a process fordeveloping photoconductive images by electrolytic oxidation.

It is a further object of this invention to provide an electrolyticmethod for preparing litho-graphic plates.

Still another object of this invention is to provide novel lithographieplates.

In accordance with this invention, a photosensitive sheet is made ofphotoconductive material, usually in powdered form, bonded to anelectrically conductive backing, preferably by means of a sui-tablenonconductive binder material. After exposure of the photoconductivelayer to a light image, the electrically conductive backing of thephotosensitive sheet is connected to the negative pole of a directcurrent source and electrolyzed in the presence of a conductive salt ofa reducible metal. During electrolysis, the metal of the conductive saltis plated out as a conductive metal-lic layer or coating on the lightstruck areas of the photoconductive surface. If

lUnited States Patent O iddi@ Patented Aug. 3, 1965 this electrolysis iscarried out with the electrically conductive backing, connected as theanode, i.e. if the electrodes are reversed, the rectification effect orselective resistance of the photoconductive layer greatly hinders andsubstantially prevents free current passage in the opposite direction.However, it has now been found that the presence of free metal, which isplated out on the photo-'conductive surface in a differential patterncorresponding to the original light image, unexpectedly destroys therectification effect of the photosensitive sheet in those areascontaining the free metal and permits current passage in bothdirections. Upon reversing the electrodes and making the electricallyconductive backing the anode, the conductive image can be developedelectrolytically by an anodic process or reverse current process.

Among the developers which may be used in this anodic process aresubstances capable of changing color Value on oxidation, such as theleuco form of vat dyestuffs used in the dyeing of various commercialfibers. For example, if the anodic process is carried out with Indigowhite in con-tact with the metal plated photoconductive surfaces, theanodic reaction oxidizes Indigo white from its colorless leuco form toinsoluble colored Indigo in the conductive surface areas. The finalvisible image is found to be stable except for the tendency to fadeslowly, probably because of the oxidation of the leuco dye on exposureto air. These dyestufis can be incorporated into the electrolyte or maybe coated on metal plated photoconductive surface prior to electrolysis.

Still another developer material that may be employed in the anodicdevelopment process is lthe colored anion, as exemplified by the acidtype dyestuffs. By carrying out the electrolysis with the metal platedphotosensitive sheet as anode and with an acid type dyestuff in theelectrolyte, the colored anions of the acid type dye migrate selectivelyto the conductive metal plated image areas and are deposited thereon,thereby coloring the light exposed and metal plated surface areas. Thesedyes are commonly marketed in the form of a salt of their sulphonicacid, usually the sodium salt. Illustrative of such developers are thenitro dyestuffs, such as Naphthol Yellow ((11.9), the mono-alodyestuffs, such as Fast Red (C.I.176), the dis-azo dyestus, such asCrocein Scarlet (C1277), the nitro dyestufs, such as Naphthol Green(C.I.5), the triphenylmethane dyestuffs, such as Wool Green (C1. '7 37),the xanthene dyestuffs, such as Brio Fast Fuchsine BL (C1758), theorthraquinone dyestufts, such as Solway Blue SES (01.1053), the azinedyestuffs, such as Azocarmine (CLSS) and the quinoline dyestufis, suchas Quinoline Yellow (C1801). Although some color is often deposited inthe background areas when the colored anion containing electrolyte isbrought into contact with the metal plated photosensitive sheet surface,the depth of color is significantly greater in the light struck, metalplated areas, and the contrast can be controlled by selection of thecolored anion, concentration of colored anion in the electrolyte,duration and conditions of the electrolysis, etc.

in some cases, during the above reverse current development some of themetal plated on the light struck areas may be reoxidized by the anodereaction, with a resultant decrease in current flow. However, this doesnot significantly affect the color development, and for certain purposesis desirable in order to improve the quality or intensity of the finalcolor.

It is also within the scope of this invention to incorporate colored oruncolored negatively charged particles into the electrolyte and carryout electrolysis using the metal plated phot-osensitive sheet as anode,as earlier described. These negatively charged particles are preferablyin suspension or dispersion and may constitute a latex, such as a latexof polyethylene, polypropylene, etc.,

etc. Generally, those polymer latices stable in alkaline -V mediacontain negatively charged particles and are therefore operable in theinstant electrolysis. The charge on such particles is readily'determinedby well knownV methods.` Y f During the` electrolysis the -negativelycharged particles are deposited selectively on the metal plated orconductive areas of the photoconductive layer. When such negativelycharged particles constitute materials which are hydrophobic in relationto the photoconductive surface Aand are therefore selectively inkreceptive and water rejecting, e.g. polyethylene, polypropylene, etc.,the surface of the resulting polymer coated sheet displays inkreceptive, hydrophobic properties in the polymer coated, light strucklareas, and relative ink rejecting,jhydrophilic background areas. Othernegatively charged particles or ions which Kdevelop -a layer or film onthe light struck image areas that display ink receptive propertiesdifferent Y from, i.e. relative'to, the background or non-light struckareas, wouldalso produce a similar result. The selectively coatedphotosensitive sheet having varying degrees of kink receptivity,`corresponding to theil original light image, provides a simple,inexpensive and effective lithographic plate and produces outstandingreproductions when used in conventional lithographie processes, e.g. theMultilithprocess In certain instances itmay also be desirable toincorporate certain iller materials in the negativelycharged particles,thereby to alter the physical properties of the deposited polymer layer.These fillers need not carry an electrical charge, since the negativelycharged particles of polymer serve as carrier for these ller materials.Suitable fillers include colored pigments, carbon black, silica, etc. Y

FIGURE 1 illustrates the above-described novel ,lithographicsheet-obtained in accordancewith this invention'.

In the above process, the deposition'of freefmetal can be effected byelectrolysis of an organic or inorganic conducting salt of a reduciblemetal, such as silver nitrate,

nickelous chloride, copper sulfate, etc. Any salt of an electrolyticallyreducible metal, which metal is electricalily conductive, may be used.Moreover, the unreduced metal salt can be incorporated in'or'on thephotoconductive material and thereafter be reduced in' situ byV elecitrolysis. The actual mechanism by-which this deposited freemetaldestroys the rectification effect ofthe photoconductive material. is notfully understood;l Y

The receptor sheet which is exposed to the light image and on which thereduced metal is differentially deposited t contains an electrically.conductive base, such as Ymetal foil, upon which a photoconductivelayer, having a photoconductivity value of at least about 10-7 -mho/cm.and a dark conductivity value not greater than about onetwentieth of thephotoconductivity value, is placed or bonded. Such sheets are describedin greater detail in SN. 692,529, led October 28, 1957. In bondingthephotoconductive material to the electrically conductive base, variouswater resistant, flexible adherent film form- W king polymersrcan beused, provided the polymer isV light in color and does'notadverselyaffect the light sensitivity of the photoconductive material. Y Suchpolymers include, for example, a 30:70 copolymer of butadiene andstyrene (Pliolit'e S-7 solution, 340% solution in toluene),Ypolystyrene, chlorinated rubber, rubberhydrochloride,poly-y vnylidenechloride, nitrocellulose, polyvinyl butyral, silicones (e.g., D-C 803silicone solution, 50% solution in xylol of alkyl amyl silicone resincapable of curing one hour at 480 F.- to a hard .and somewhat brittlepolymer), etc. Polymers which are dissolved or softened by water, orwhich are dark in color, or insoluble in commercial solvents, orreactive with the photoconductive material, or which readily wet thephotoconductive particles, are

found to be less desirable. Thus, polyvinyl alcohol,y

polyacrylie acid, shellac and sodium carboxymethyl'cellulose aregenerally not employed as binders for the light- Y values in coatedrilrn form and to produce receptor sheets suitable forr use in theinstant invention. Mixtures of these photoconductive materials may alsobe employed. Generally, lzinc oxide is preferred. To improve or enhancethe sensitivity ,ofthese photoconductive materials in certain visibleand non-visible areas of the light spectrum, dyesensitizers such' asAcridine Orange, Fluorescein,. Eosin jY, Rose Bengal, MethyleneV Blue,etc., are preferably admixedin smallquantitieswith the photoconductivepowder.

Metal foil or sheet providesa suitable electrolytically conductivebacking. Metal' conductors, such as aluminum, chromium, nickel andcopper, are suitable for the electrically conductivebacking and mayadditionally be placed on the surface of a nonconductive `supportingsheet, c g., 'by vapor deposition, lamination,` etc. When thephotosensitive sheet is immersed in the electrolyte during electrolysisthe exposed conductive backing must beinsulated from the electrolyte,e.g. by anonconductive surface coating such as plastic, etc. s f Thefollowing examples are illustrative and are not to be construedaslimiting the` scope of theV instant invention. f v

Examplel `A suitable light-sensitive sheet material was first prepared.A flexible film of transparent cellulose acetate having a thickness ofabout l0 mils (0.010 inch) was first metallized on one surface, by vapordeposition ina vacuum, with an extremely thin 'coating ofY aluminum.VThe coating was found to have arsurface resistivity of labout 200 ohmsper square, and transmitted-about 55% of incident light in the visiblerange. Over this metal layer was then applied a suspension of 48 partsbyweightV of Merck & Companys Reagent-Grade zinc oxide microcrystals ina solution, in 48 parts toluene, of 4 parts of Pliolite S-7 resin, laresinous copolymer of about 30 parts butadiene and correspondingly about70 Yparts styrene, serving as a binder, the mixturehaving been ground ina ball mill until smooth. After drying, the firmly bonded smooth lwhitecoating was'found to be between 0,3 and 0.6 mil in thickness. The sheetmaterial was highly water-resistant. Y l Y Sheet material preparedasjust described was suspended in -a transparent glass cell containing asolution of 28 grams of copper sulfate m200 ml. of water. A at electrodeof slightly larger area, in this case a copper plate, was suspended inthe solution facing and somewhat removed from the coated surface of thesheet material. A light-image was focused on the uncoated surface of thesheet through the -glass wall of the cell, the source of the light'beinga 10G-watt bulb and providing an intensity of aboutr 70 foot-lamberts.vExposure was maintained for about 5l seconds. A source of potential wasthen connected across the copper plate and the conductive aluminum layerof the sensitive sheet, the latter being connected to the negative pole,and a lcurrent of `about 15 milliameperes was passed through the systemfor about 3 seconds. vThe sheet was withdrawny and rinsed, and was foundto have a negative reproduction of the light-image on the sensitivecoating. Non-illuminated 'areas of the sensitive coating remained white,while the exposed areas were darkened by deposition of metallic copperthereon. Equally effective copy was obtained by exposing the coatedsheet to thelight-image underldry conditions, and then promptlyimmersing the rsheet in the `electrolytic cell and electrolyticallydeveloping the image in the manner described. Y

Silver nitrate solution was substituted for the copper sulfate toprovi-de equally effective ivm-age development. Nic-kelous chloride isalso eiiective, and is improved by the addition of sodium thiosulfate. Aparticularly effective developing solution contains nickelous chlorideand 5% sodium thiosulfate,

'Ilhe ratio of pigment to binder in the light-sensitive coating waseiectivelly varied over wide ranges. At 12 parts of Zinc oxide to one ofresin, as in the specific formula just given, the white areas of theprint are sometimes found to contain dark spots, indica-ting non-uniformor insufficient resistivity. Excellent prints are obtained at lowerratios, for example at 8:1 and at 4:1. Somewhat less effective printsare obtained a-t 3:1 ratios of zinc oxide and Pliolite, resin, and at2:1 the lightsensitivity is inadequate and the results are decidedlyinferior. These ratios may be specifically difiere-nt with otherspecili-c oxides and resins but will serve to illustrate p a generallydesirable range.

Electrically conductive glass plates have been substituted for thepartially transparent metallized cellulose acetate film as a `carrier orbase for the light-sensitive coating. A glass having a surface layerhigh in stannic oxide, h-aving a surface resistivity of about 600 ohmsper square and a lig-ht transmission of at least about 90%, has provenuseful, although somewhat lower resistivity is preferred.

The sensitive surface of such transparent photosensitive coated pla-tesis effectively exposed to the light-image through the transparent plateand simultaneously electrolytically developed, as described in theforegoing example. rThese plates may ,alternatively be first exposed tothe light-image and then, without further irradiation, transferred tothe developing station and separately developed, lthe iight-memory ofthe zinc oxide coating being sufficient to maintain the necessaryconductivity at the irradiated areas. The latter procedure is equallyeffective on fully opaque plates such as metal plates coated with thesensitive zinc oxide coating.

Opaque plates have been simultaneously exposed and developed bysubstituting a copper wire frame for the Icopper plate of Example 1 andthen exposing the sensitive surface of a coated metal plate to alight-image through the trame while carrying out the elect-rolysis asbefore. Where the plate area is too large for uniform electrolysis inthis manner, a screen is provided Iin place of the frame, and the screenis moved steadily during electrolysis so as to avoid producing a visibleshadow pattern on the sensitive sheet.

Example 2 A photosensitive sheet having a 5 mil conductive aluminumbacking and a coating thereon of zinc oxide and iPliolite S-7 resin(about 10:1 respective weight ratio) was exposed to a light image for aperiod of seconds, using .a 100 .Watt light source to project the imagefrom a positive mircoiilm tra-me to the pho-toconductive surface of thesheet. The exposed surface was then brought into contact with a solutionof 10% by weight of nickelous chloride and 5% by Weight of sodiumthiosulfate and electrolytically developed at an impressed potential ofbetween about 30-40 volts D.C., the conductive .aluminum .backing beingconnected to the negative pole of the current source and being masked onits exposed face with a plastic covering, .as described in Example 1.

A-fter a period of 15 seconds the electrolysis was stopped and theexposed surface Was rinsed with cold water to remove residual metalsalts, A negative reproduction of the light .image was formed, thenickel having been reduced and deposited as the free metal on the lightstruck areas of the photosensit-ive surface. Conversely, if a negativelight-image is used, a positive reproduction iS formed.

The sheet was then suspended in a 1% solids aqueous polyethylene latex[A-C Polyethylene 629, Allied Chemical and Dye Company, MP. Z13-221 F.,penetration gm., 5 sec., 77 F.) of 3-6, aoid No. 1-417, color less thanl NPA]. Using the conductive aluminum backing sheet as the positive poleand masking the exposed aluminum backing with a plastic covering, thepolyethylene latex was electrolyzed at a potential of labout 30-40 voltsfor about l5 seconds. Polyethylene was deposited selectively on thoselight struck areas having a free meta-l deposit, producing apolyethylene image which had greater .ink receptivity relative to thezinc oxide background areas. When used as a lithographc plate negativeprints were obtained. If the original light-image is negative, the iinallithographie prints are positive.

Other polymeric latices which are stable in alkaline media and whichcontain negatively charged particles can be similarly employed,including polytetraltuoroethylene, `synthetic rubber, eg. Ohemigum latex24S-B (butadieneacrylonitni-le, non-staining, oil resistant,vulcanizable synthetic rubber latex supplied by the Goodyear Tire andRubber Company), polyvinyl acetate, polystyrene, Pliolite, rubber,polyvinylidene chloride (Saran), etc- The above described lithographie`plate was installed on a Multilith lithographie machine and providedover 250 copies with excellent contrast and definition. An acid fountainsolution containing canboxymethyl cellulose and various inks were used.Final prints were dried at 200 F. for 15 minutes.

Various other embodiments and variations within the scope of thisinvention will be readily apparent to persons skilled in the art.

Having thus described my invention, I claim:

1. A novel lithographic sheet which comprises .an electrical-1yconductive backing, a photoconductive layer bonded to one contiguouselectrically conductive surface Ithereof, a free metal image on selectedsurface portions of said photocond-uctive layer, and a polymer coatingon the surface of and coextensive with `said free metal image, thepolymer off said polymer layer being capable of forming a latex in whichthe polymer particles bear a negative charge and being selected toprovide a polymer coating having different wetting characteristics withrespect lto lithographie ink and fountain solution than the otherportions ofthe sheet surface,

2. The lithographie sheet of claim 1 in which said polymer ispolyethylene.

References Cited by the Examiner UNITED STATES PATENTS 2,957,765 10/60Resetich lOl-149.2 X 2,993,787 7/ 611 Sugarman. 3,107,169 10/63 BornarthlOl-149.2 X

WILLIAM B. PENN, Primary Examiner. DAVID KLEIN, Examiner.

1. A NOVEL LITHOGRAPHIC SHEET WHICH COMPRISES AN ELECTRICALLY CONDUCTIVEBACKING, A PHOTOCONDUCTIVE LAYER BONDED TO ONE CONTIGUOUS ELECTRICALLYCONDUCTIVE SURFACE THEREOF, A FREE METAL IMAGE ON SELECTED SURFACEPORTIONS OF SAID PHOTOCONDUCTIVE LAYER, AND A POLYMER COATING AN THESURFACE OF AND COEXTENSIVE WITH SAID FREE METAL IMAGE, THE POLYMER OFSAID POLYMER LAYER BEING CAPABLE OF FORMING A LATEX IN WHICH THE POLYMERPARTICLES BEAR A NEGATIVE CHARGE AND BEING SELECTED TO PROVIDE A POLYMERCOATING HAVING DIFFERENT WETTING CHARACTERISTICS WITH RESPECT TOLITHOGRAPHIC INK AND FOUNTAIN SOLUTION THAN THE OTHER PORTIONS OF THESHEET SURFACE.